The present invention relates to a method of controlling the rotational speed of a rotary shaft by feedback.
To control by feedback the rotational speed of a crank shaft of an automobile or the rotational speed of a drive shaft coupled to the drive wheel of an automobile, the actual rotational speed of the rotary shaft is, first, detected, and then the difference between the detected rotational speed and a desired rotational speed is calculated. Thereafter, the opening degree of the throttle valve or other speed control valve is adjusted in response to the calculated difference to control the rotational speed of the engine of the automobile. In general, the rotational speed of the rotary shaft is detected by generating a pulse each time the shaft advances by a predetermined angle. These pulses pass through an open a predetermined number of pulses, for example, 12 pulses, are generated, and by to count using a binary counter, the number of clock pulses that pass through the gate while this gate is open. To control the rotational speed depending upon the difference between the detected rotational speed and the desired rotational speed, the actual rotational speed must be detected with good precision. According to the above-mentioned conventional method, it is possible to detect the rotational speed with relatively high precision. However, the following problem occurs: since the content of the binary counter is inversely proportional to the actual rotational speed, it is necessary to execute a division operation with respect to the content of the counter in order to obtain a signal proportional to the actual rotational speed. The division operation usually requires an increased number of digits to secure the same precision as the other arithmetic operations and hence, requires an increased number of operation steps, causing the operation time to take longer.